As with the commits earlier, this also adds const qualifiers where
expected. However, the const variables are casted to non-const variants
so they can be passed to glib functions that take non-const variants but
expect const-like input.
Add helper functions and macros for managing and drawing OpenGL ES 3.
It will be used for blitting framebuffers between multiple GPUs in
hybrid GPU systems.
https://bugzilla.gnome.org/show_bug.cgi?id=785381